Apparatus and method for remedying cross talk



March 2 6, 1935. R, N- HUNTER 1,995,454

APPARATUS AND METHOD FOR REMEDYING CROSS TALK Filed Oct. l2, 1953 sw l ATTORN EY Paienied Mar. 26, 1935 UNITED STATES PATENT oi-#rica 1,995,454 t t Y APPARATUS AND METHoDroR REMEDYING f caoss TALK v Russell Normanrlunter', Westfield, N. J., assigner to American Telephone and Telegraph Com-f pany, a corporation of New York Application-october 12, 1923, serial No. 693,373' 8 claims. (o1. 17a-7s) VAn object of my invention is to provide a method and suitable apparatus for eliminating crcsstalk signaling circuits. Another object is to correctl for crosstalk in'a system of trans- 5 mission lines Vexisting side by side. Other obof themfto correct for'crosstalkboth ways. Still jects are toprovide a system of 'transmission lines with connections across between any two another object is to provide c'rosstalk correcting means from one pair to another that'iseifective only in`one of the two waysbetween the vtwo pairs. Still'anoth'er object is to provide cross- 'talk correctingV means between two pairs that is independently adjustable to 4correct Yfrom the Iirst-pairto the second, and from the second to the first. Allof'these objects, and various other objects and advantages of my invention, will YbeaY come' apparent in connection with the following disclosure of a limited number of specific examples of practice according V to the invention which I have 'chosen tobe presentedv in this specication.' It will be understood that this disclosure has relationimore. especially 'to these particular examples of the invention, Vand that i the scope of the invention'willbe indicated the appended'claims. 1 Referring to the drawing, `Figure 1 'is' a diagram of a transmission system in Awhich my invention may beembodied; Fig. 2"isadiagramY showing two opposite one-way mutual inductance balancing connections Vbetween two, conductor pairs; Fig. 3 is asimilar diagram, except that the connections involve capacity'insteadof mu-` tual inductance, and Fig. Llisa diagram showing similar connections associated' with the reg`` ular line repeaters on the two pairs;v

When two'signaling circuits extend closetogether the existence'ofsignaling currents in one circuit may have a tendency to set up corresponding currents in the othen circuit; this phenomenon is called crosstalk, The joureffects on oneA conductor pairfon neighboring pairs are for the most part balanced' out. In

open Iwire lines the conductors 'of' each pairare systematically transposed soy that the influence upon neighboring pairs, and the influence fromv them, are for'the most part balanced out. How.-

ever, in many Vcases of longV transmission lines,

eitherin vcables orin open wirelines, in spiteof the twisting or transposing of the conductors, there will be some accidental dissymmetries and, therefore, some degree circuit and another, unless further corrective adjustment is made. InV one aspect my invention relates to. making compensating adjustments toremedy. the crosstalk between the neighboring circuits v.of long cables or multiple open wire lines, and more particularly, 4to ycases in which different adjustments areV required to. compensate the rcrosstalk one wayl as between' two circuits and the crosstalk the opposite "way between them.A

To illustrate the nature of' my invention, 1 will now describe 'the apparatus and the ,procedure'for'correcting crosstalk between twov rep-l resentative conductor pairs' Vof a' long cable. Referring to Fig. 1, this shows several con'duc'` tor pairsv `or circuits in a cable which may be,

say 25 miles long. At each end there maybe repeaters and further 25 mile cable lengths eX- of crosstalk between one` tending therefrom. By the twisting of each"` A conductor pair. inv the 25 mile'length of cable, most of the mutual inductance and mutual capacity between any two circuits will be canceled out. However, there will sometimes be a little unbalanced mutual inductance effect between." the circuitsY at a certain location along their length which maybe represented as m1, 'in Fig.

1, and likewise,` at such'a vplace, there, may be alittle unbalanced ,mutual capacity between the two circuits 'represented Vby C1. Whereas, the couplings represented by m1 and c1 aretoward the left as viewed in Fig. 1, there maybe such couplings'anywhere along the length (ofthe cable as represented by m2 and c2 at the right.. assuming that signaling currents are transmitted from the left on the upper circuit, these couplings,m1, c1, m2 and cz, will give rise to a small degree of corresponding currents inducedN in the lower circuit, which .will become' effective.' of fthe, lower The magnitude of these receivedvcrosstalk currents will depend on the 'net resultant.` effect of the inductances m1 and m2 andthecapacities c1 and c2. Y It may be assumed thatl as crosstalk at the right-handend circuit. 1

the two circuitsare terminated each in its char- Y acteristic' impedance, but, in the presentk case, it is not necessarily assumed that 'the' two cir. g

cuitsare alike, andifthey'are essentially'difl-'j ferent the net overall crosstalk effect from the first circuit into the second, may be different from what it is from the second to the rst.

At a convenient place along the linea panel for adjusting elements is provided such as P inFig. 1, and on this panel reactance elements` are mounted and connected from one side of one pair to a sideof the other pair so as tojcompensate the crosstalk. If theV uncompensated coupling, such as shown by the ms and the cs in Fig. 1, is a mutual inductance couplingit will require an opposite'rnutual inductance bal.- ancing `on the panel P. 'The appropriate con- Y nections are shown in Fig. 2. Each of the' symbolic elements B is a one-way. device such asl a two-electrode vacuum tube `thatpermits currents to flow only in the directionof the respective arrow. To compensate-forthe resultant in-A ductive coupling from the pair 1 2 to the pair 3 4 the mutual inductive connection M1 is provided with connection to the appropriate side of ea`ch-pair-to provide theneces's'ary compensation; Itis, of course, necessary that proper'phase v is 'a capacity'effect, then itwiirrequire capacity" relations of the various currents shall be maintained.` For this purpose the resistances representedlby R1 andfPt; should be highconipare'd It willv be seen that there aretwo'connections in Fig. 2 in opposite directions' and quite`ihdependent'of one another byvirtue of thefrespec;

tive opposite one-way" devices 'BI interposed' in? each connection.

Il the overall vunbalance between the two lines compensation which may be provided asshowri in Issj. Thevaiues of the. capacities; cl'iandczf should be small. in order thatfthe current fand' voltage of :the circuit in' which they occur shall bejnearly' inquadrature. ',Whileitis notneces'- sary, it' may be feasible'and convenienttomake the resistances`R3 ofV Fig'. 3.Y oi the same Value a's the resistances Rif'and'Ri4 (which may be 'of equalivalue) in Figi. 2.- Theblocking drone-f way devices B should introduce negligible phase shift in Fig.' 2 and inl Figlfif 'Fomore'precise balancing it may be desirable to employ the con?` nections of Fig. 2' and of Pig. 3 simultaneously.

VThe"locationlofY the panel P alongthe sectionk n between the repeaters represented in Fig. l, may v be at' any convenient place. There may be a certain advantage in having it halfway between the repeater stations, for if thereareimpedance ir,- regularities at the ends of the section their dis-Y turbing effect may be somewhat less with the balancing panel atthis location than if it were A at "anend of the section.

The' balancing cross-connections maybe established Vat a repeater station 'witl 1o.u i';V neces-` sitatin'gtheuse of entra one-'way devices 'such' asQthe Bs in Figs.v 2 'and 3.` This Valtern'ative '"I Scheme .is iniceieiin inedita@ @.Fig- @what those teures! Rsistancesimayjte,combinerwitn the coils M1 and M2 and the condensers C1 and C2 are connected diagonally across, as shown. Adjustments of these connected elements may be made to reduce the crosstalk in the preceding repeater section, that is, the section lying to the 5 left, as viewed in Fig. 4. .This method of connection shown in Fig. 4 is not purely one-way betweenthe pair 1 2 and the'pair {1 4, but the crosstalk coupling caused'by the connections M1 and C1 from the pair 1 2 to the pair 3 4 will exceed, by the sum of the gains of the two repeaters, the coupling through the same elements `from 'the pair 3 4A to the pair 1 2. Accordin'gly,`unless the repeater gains are exceptionally lowthis coupling M1 C1 may be regarded as substantially enel-way; that is, from the pair 1 2 to 'the pair' 3 4. "In case the repeater gains should be so low that the eiect could not be said to be substantially one-way, loss pads could. be introduced as at X1 and X2 and the repeater gains increased sov4 that the substantial. onevway effect would be established.

AJust as the coupling elements M1 and C1 corn-l pensate Vfor'ero'sstalk from the pair 1 2vr intothe pair 3 4, so the elements M2 andCz, compensate for the crcnsstall'zr from 3 4 into 1 2. Thisgmeth, od requires that the gainsV the repeaters slriall be approximately constantbw not necfe arly Y equal. It, alsorequresthat the Phase Shftin the repeatersshallbe Small; or, if, itis. not Small; it should at least be constant so that, allowance for it may be made by the proper choice 'of bal,- aning, units;- As in, the diSSuSSel` Colnnection withv Figs. 2 vand 3, the imperiancesofl the two coils oi Mii orMz' should be4 small, and'fthej values of the .capacities C1 and C2. should be Small. `In praticeithese values will neQeSSarily; be smallV because the crosstalk. Coupling which they introducev will be amplied by the samer, one repeater. l. s

The methods that `have, beendescribedin,connection withfFi'gs. 2 and 3, vand connection with Fig. 4, arenot limitedtothe mutualind vcr.- tance, and capacity balancing, ShQwn inl either the capacity v'units" orthel mutual i.r idul tance units to producepd'esired'phasashiits,`or more complicated networksrnay be :made up tolt, cases,` wherethe crosstalk` varies` in Ia, lesssmple relation:with,frequencythan has` been assumed..

VThelone-.way devicesyB VQfFigns. 2 and ,.3 may., be ordinary 'one'way repeaters, Abut they 'need'. not provide ampliiiicationf since, Y, their only., pur,-V pose is to ,block cnr'rentsj'in one. of ,theftwo .opposite directions.v Repeate'rs; I with negativa.' feeld'-,v back may be suitablefor'this pil'rpose being ade. justed to giveconsiderable loss and'\.to. bex very stable'under such conditions.

What S Claimed ist 1. Means for neutralizingcrosstalleiectsbe; tween two dissimilar conductor pairscomprising a connection` from'onasidebt' one pair to 4lone j side of the other pair;an,impedance` element that connectionanda .onerway device also cont-, prised in thatcon nectin.t i

' 2. Means for neutralizing crosstall effects ,be-4 tween twvdissfimilar `'conductor pairs comprising two conneeti'onsffrompne side of ong pair to. nef side ofthe', other pair, r'espectiveimpedance Aele-;v ments in'thse, connections .andresH K positefvsoneway devieesfalsoco'mprised v connections.; J Y Y '3- TA' pllrlit 0f dissimilar. @engager reirse iendingisdbrs @amatagaeta@ col ments located at a point along that length and each connected across between a conductor of one pair and a conductor of another pair to neutralize the crosstalk between suchpairs, and oneway devices associated with said reactance elements so that the neutralization shall be only one way therethrough from pair to pair.

f5. At a repeater station, lines and respective repeaters, means for compensating crosstalk from one line to another comprising a 'connection from the output side of the repeater of the one line to the input side of the repeater of the other line and an adjustable impedance element in said connection. 1

6. At a repeater station, lines and respective repeaters, means for compensating crosstalk from one line to another comprising a connection from the output side of each of two repeaters to the input side of the other of those repeaters, and adjustable impedance elements in those connections.. v

7. Means for neutralizing crosstalk eiects be tween two dissimilar conductor pairs comprising a connection from one side of' one pair to one side of the other pair, an impedance element in that connection, and means to make the effect therethrough greater from one pair to the other than from the other to the one.

8. Means for neutralizing crosstalk eects be-v tween two dissimilar conductor pairs comprising Y two connections from one side of one pair to one side of the other pair, respective impedance elethrough, these directions of greater effect being v20 opposite in the two connections.' f

' RUSSELL N. HUNTER, 

